Structural members in aircraft, ships and other equipment may be exposed to considerable stress from a variety of sources, including static and dynamic loading and temperature and pressure variation, and may be subject to further degradation as a result of exposure to corrosive materials and other environmental factors. As a result, the structural members, typically constructed of metal, ceramic or graphite/epoxy laminate materials, may develop structural defects, including the creation or propagation of cracks, delaminations or other defects. These defects may contribute to the catastrophic or other failure of the structural member and hence may pose a threat to human safety or lead to overall equipment failure. As a result, it is important to be able to detect structural defects so as to provide an opportunity to replace or repair structural members before structural failure of the member.
Detection of structural defects in aircraft and other equipment has conventionally been accomplished by periodic visual inspection of structural members exposed to stress. Visual inspection techniques are limited, however, by the need for disassembly to gain access to hidden members, by the difficulty of detecting small defects and by the high labor costs associated with visual inspection.
To overcome the limitations inherent in visual inspection, alternative non-destructive inspection techniques have been developed, including ultrasonic inspection, electrical eddy current inspection, nuclear magnetic resonance (NMR) and radiographic inspection. These alternative techniques, however, typically require sophisticated and costly inspection equipment, highly-skilled technicians, significant disassembly for access, and/or substantial equipment down time.
To avoid the aforementioned limitations, techniques for the detection of cracks and other structural defects have been developed which rely on the deposition of very thin electrically conductive paths or traces ("crack wires") on or within the structural member of interest. See, for example, conventional crack gages such as the Micro-Measurements Crack Detection Gage Model CD-23-50A, Micro-Measurements Crack Propagation Gage Model CPA01 and multiplexable Micro-Measurements Crack Propagation Gage Model CPD01. As defects develop or propagate through the member and across a crack wire, the crack wire is broken. Conventionally, each end of the crack wire extends to the edge or surface of the structural member. In order to detect a structural defect that has broken a crack wire, each end of the crack wire can be physically contacted, such as with a pair of probes, in order to perform a continuity check or resistance measurement.
A number of active monitoring systems have been developed to overcome some of the limitations of inspection-based techniques. For example, an active monitoring system based on mechanically exciting a structure and comparing the measured mechanical response to the excitation to an expected response is disclosed in U.S. Pat. No. 5,195,046. These and other active monitoring systems incorporate complex electromechanical components that require battery power and are typically costly and susceptible to failure.
While crack wires, active monitoring systems and other similar defect detection techniques offer advantages relative to the other aforementioned conventional techniques, significant disassembly may still be required, physical contact with each crack wire must be made, a battery or other source of energy is required, detection hardware is complex and costly and substantial labor costs and equipment down time may still be incurred. Therefore, while a number of defect detection techniques have been developed, it is still desirable to develop improved defect detection techniques which do not suffer from the inherent limitations imposed by prior defect detection techniques, such as disassembly of or other physical contact with the structural member, battery requirements, extensive equipment down time and high labor costs.